linux-kernel - Re: [PATCH] svm/avic: Fix invalidate logical APIC id entry

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Message-ID: <87ef6zcb7i.fsf@vitty.brq.redhat.com>
Date:   Fri, 22 Mar 2019 14:04:01 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     "Suthikulpanit\, Suravee" <Suravee.Suthikulpanit@....com>
Cc:     "joro\@8bytes.org" <joro@...tes.org>,
        "pbonzini\@redhat.com" <pbonzini@...hat.com>,
        "rkrcmar\@redhat.com" <rkrcmar@...hat.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm\@vger.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH] svm/avic: Fix invalidate logical APIC id entry

"Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com> writes:

> Only clear the valid bit when invalidate logical APIC id entry.
> The current logic clear the valid bit, but also set the rest of
> the bits (including reserved bits) to 1.
>
> Fixes: 98d90582be2e ('svm: Fix AVIC DFR and LDR handling')
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
>  arch/x86/kvm/svm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 7a4ce1a22ca0..f4fb766e474c 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -4640,7 +4640,7 @@ static void avic_invalidate_logical_id_entry(struct kvm_vcpu *vcpu)
>  	u32 *entry = avic_get_logical_id_entry(vcpu, svm->ldr_reg, flat);
>  
>  	if (entry)
> -		WRITE_ONCE(*entry, (u32) ~AVIC_LOGICAL_ID_ENTRY_VALID_MASK);
> +		WRITE_ONCE(*entry, (u32)(*entry & ~AVIC_LOGICAL_ID_ENTRY_VALID_MASK));

I'm not sure how important this is, but this change creates a tiny time
window between reading from *entry and writing there. Should we use
atomic bitops instead?

>  }
>  
>  static int avic_handle_ldr_update(struct kvm_vcpu *vcpu)

-- 
Vitaly